This invention relates to improvement in gear assemblies, and in particular to electrical power assisted steering assemblies which incorporate a worm and wheel gear assembly for transferring torque from an electric motor to a steering column or output shaft operatively connected thereto.
It is known to provide a power steering system for a vehicle comprising an electric motor having a stator and a rotor, an input shaft operatively connected to the rotor and adapted to rotate therewith, an output shaft associated with a steering column, and a gearbox adapted to transfer torque from the input shaft to the output shaft in response to a measure of torque in the output shaft produced by a torque sensor. The motor is typically operated to apply an increasing torque to the output shaft as the measured torque increases, thus applying an assistance torque which helps to steer the vehicle.
In a simple arrangement, the input shaft defines a worm gear, and the output shaft is provided with a wheel gear which is adapted to mesh with the worm gear. Whilst such a system is relatively effective, there exists a problem with noise and vibration due to incorrect meshing between the worm and gear wheel. This incorrect meshing may arise due to manufacturing tolerances, thermal changes in dimensions, distortion due to torsional loads and wear during service.
We are aware of U.S. Pat. No. 4,967,859 which discloses an electric power apparatus comprising all the features contained in the pre-characterising clauses of claims 1 and 18. Specifically, the apparatus of U.S. Pat. No. 4,967,858 does not include any form of biasing for the shaft.
We are also aware of EP-A-0420131 which discloses a rear wheel steering apparatus in which a worm shaft extending from a motor is biased towards a worm wheel by a resilient means in the form of helical spring.
In J-A-62255618 a resilient element constituted by leaf springs has a plurality of drive faces which co-operate with drive faces defined on a rotor and on an input shaft.
According to a first aspect, the invention provides an electric power assisted steering system comprising a housing, an electric motor fixed relative to the housing having a stator and a rotor, an input shaft operatively connected to the rotor, an output shaft operatively connected to a steering column, and a torque sensor adapted to produce an output signal indicative of the torque in the output shaft, the motor being adapted to apply a torque to the output shaft dependent upon the output signal from the torque sensor through a worm gear provided on the input shaft which is adapted to mesh with a wheel gear on the output shaft, the steering system being characterised by further comprising a first bearing means which supports the input shaft relative to the housing at its end distal from the motor and a resilient biasing means adapted to act upon the first bearing means to bias the input shaft towards the wheel gear.
Preferably, the input shaft is biased in a tilting movement which is centred at a second bearing means which supports the input shaft relative to the housing at its end adjacent to the motor.
The biasing means may be adapted to apply a sufficient biasing force to the first bearing means to maintain a fully meshed engagement between the teeth of the worm gear and the teeth of the wheel gear over a predetermined range of torque values carried by the wheel gear. This helps to prevent gear rattle when driving straight ahead or on rough roads by ensuring both sides of the engaging teeth on the worm and wheel are in contact at substantially all times over this range of torques. Because the arrangement increases quiescent friction in the gearbox it is important to maintain control of the force applied by the biasing means over the full range of the input shaft that is required. Therefore the biasing means must have a low spring rate.
The provision of the biasing means allows a controlled biasing force to be applied whilst permitting sufficient tilting movement of the input shaft to compensate for changes in dimensions due to manufacturing variations and temperature changes etc. The maximum torque value up to which the fully meshed engagement is effective is carefully chosen (by compromise) to avoid excessive friction.
The biasing means may comprise a resilient spring of any type adapted to act between a portion of the housing and the first bearing means.
In some configurations, it is preferred that the resilient spring comprises a leaf spring which may be attached to the housing at a first end and act upon the first bearing means at its second end. This may engage the first bearing means at the opposite side of the input shaft to the wheel gear so as to bias the worm into contact with the wheel gear.
The leaf spring may be provided outside of the housing and the second end of the leaf spring may pass through an opening in the housing to engage with the first bearing means. The second end of the leaf spring may carry a seal which seals with the opening through which it passes.
The input shaft may be directly connected to the motor rotor. It may extend continuously through the rotor.
The input shaft may be operatively connected to the rotor through a flexible coupling which allows the worm to tilt without movement of the rotor.
The flexible coupling may comprise a resilient element, for example of rubber. The motor rotor may be adapted to apply a drive force to the resilient element through one or more circumferentially spaced radially extending surfaces of the resilient element. The resilient element may in turn be adapted to apply a drive force to the input shaft through one or more other circumferentially spaced radially extending surfaces of the element. The element may comprise a spider shape having a multiple of arms presenting a number of radially extending, circumferentially spaced drive surfaces.
Where the input shaft is connected to the motor rotor by a flexible coupling, a first compression means may be provided between the housing and the first bearing means at the end of the input shaft distal from the motor which applies a compressive force onto the input shaft to bias it towards the motor rotor. It may comprise a coil spring. Its function is to prevent noise and vibration due to axial free play in the second bearing means. In its compressed state, there should be free space between adjacent coils in order to avoid frictional resistance to the tilting motion of the input shaft.
A second compression means (such as a coil spring) may also be provided between the end of the input shaft adjacent the motor rotor and the motor rotor. This may be provided in a cup formed on an end of the input shaft whilst a pin extending about the rotational axis of the motor rotor and forming a part of the rotor projects into the cup to engage the spring.
Thus, whilst the first compression means biases the second bearing means through the flexible coupling, the second compression means biases the rotor directly through the pin. There must be a difference between the forces provided respectively by the first and second compression means which is sufficient to bias the second bearing means in the direction of the input shaft axis by the desired amount.
In an alternative, the biasing means may comprise an annular O-ring provided between the first bearing means and the housing. The O-ring may be of rubber, and may contact an outer circumference of the first bearing means and a portion of the housing.
The first bearing means may therefore move relative to the housing against a resistant force applied by the O-ring as at least part of the O-ring is compressed. In such an arrangement, the wheeled gear or the worm gear may deliberately be made slightly oversize relative to the dimensions which are calculated according to the distance between their respective shaft axes. This ensures the O-ring is always under a small amount of residual compression.
In an alternative to an O-ring, the resilient biasing means may comprise a resilient element which is accommodated between the first bearing means and the housing, such as a rubber spacer block. The element may be disposed between the first bearing means and the housing opposite to the side of the input shaft which engages the gear wheel. The biasing means may act in compression or tension.
The second bearing means may comprise a ball bearing which is adapted to prevent radial and axial movement of the input shaft relative to the housing whilst permitting tilting movement of the input shaft against the bias force provided by the biasing means.
The second bearing means may be selected to comprise a high tolerance ball bearing assembly which is adapted by virtue of the shape of the groove in which the balls are located to substantially prevent any radial displacement of the input shaft relative to the housing as it passes through the second bearing whilst permitting the input shaft to pivot about a point on its axis which passes through a point in the plane of the second bearing means.
In a preferred arrangement, the housing defines a first portion and a second portion, the first portion comprising a housing for the input shaft having at least one pair of opposed walls, and having an opening in each wall into which the first and second bearing means are respectively provided, and the second portion comprising a housing for at least part of the output shaft having at least one pair of opposed walls, an opening being provided in each wall for receiving one or more bearings adapted to secure the output shaft relative to the housing. The output shaft is preferably mounted orthogonal to the input shaft and substantially prevented from moving radially relative to the housing.
A plastic lining portion may be provided around a circumferential outer face of the first bearing means which prevents contact between the first bearing means and the housing at excessive displacements. This helps to eliminate vibration noise due to metal-metal contact between the first bearing means and the housing.
The first opening defined in the first portion of the housing (which receives the first bearing means) may comprise an elongated slot through which the input shaft passes having semicircular end portions and a central pair of parallel sides. The spacing between the parallel sides may be substantially the same as the outer diameter of the first bearing means. The radius of the semicircular end portions of the slot may be substantially the same as the outer radius of the first bearing means. Thus, the first bearing means may be adapted to move axially along the slot but may be prevented from moving radially perpendicular to the slot.
The first opening of the first portion may comprise an annular bore having an inner surface with a diameter greater than the diameter of the first bearing means. The outer surface of the first bearing means may thus be spaced from the inner wall. The O-ring element forming the biasing means may be accommodated in this space.
A groove may be provided around a circumference of the inner wall which locates the O-ring in a predetermined position relative to the wall, the depth of the groove being less then the radial thickness of the O-ring element. In this case, the first bearing means may be displaced from its rest position against the biasing force provided by the O-ring through a distance equal to the difference between the O-ring radial thickness and the depth of the groove. Thereafter, the first bearing means is prevented from further radial displacement within the opening as it engages the inner wall of the first opening.
In a yet further alternative arrangement, the biasing means may comprise a torsion bar having a first end acting upon the first bearing means and a second end fixed relative to a portion of the housing so that the torsion bar applies a biasing force against the first bearing means.
The torsion bar may comprise a substantially U-shaped elongate rod having a terminal end portion on a first end of the rod which is bent through an angle of approximately ninety degrees relative to the remaining part of the first end and relative to the centre portion of the rod to engage with a portion of the first bearing means. Preferably, the terminal end portion acts directly on an outer surface of the first bearing means opposed to the wheel gear of the output shaft by passing through an opening channel in the housing extending radially away from the inner wall of the first opening of the first portion of the housing.
The central portion of the torsion bar may be secured to a portion of the housing through one or more clamps or shackles.
The second end of the torsion bar may rest upon an end face of a threaded bolt which engages with the housing. Rotation of the bolt within the threaded bore displaces the second end of the torsion bar relative to the housing. As the first end is engaged with the first bearing means this acts to increase or decrease torsion in the bar in a known manner, in turn to alter the biasing force applied to the first bearing means (i.e. for use when setting up).
In a preferred arrangement, a terminal portion of the second end of the torsion bar is bent through approximately ninety degrees relative to the remaining part of the second end portion and engages within a recess in the end face of the bolt. This provides a positive location for the second end portion.
In yet a further alternative arrangement, where space in the vehicle permits, the biasing means may comprise a coil spring having its axis substantially perpendicular to that of the wormshaft. The coil spring could be installed in a hole in the housing. A first end of the spring could apply force to the outer race of the first bearing means via a formed end of the spring or via a separate component placed between the spring and the first bearing means. A closure plug or plate at the end of the hole distal from the bearing means would provide a support to the coil spring and a means of sealing.
In a most preferred arrangement, the terminal portion of the second end of the torsion bar engages with a recess in the housing. This renders the arrangement non-adjustable and tamper proof which is preferable for production versions.
In a refinement, where the biasing means comprises an O-ring seal acting between the first bearing means and the housing, the centre axis of the O-ring may be offset relative to the central axis of the input shaft. This provides a different relationship between biasing force and displacement of the bearing means compared to the case where the central axes of the O-ring and shaft coincide. It is preferred that the axis of the O-ring is closer to the wheel gear than that of the input shaft where is passes through the first bearing means.
To further refine the relationship between the biasing force and displacement of the first bearing means, the shape of the O-ring groove (where provided) relative to the cross-section of the O-ring may be chosen so that the compressed portion of the O-ring just completely fills the groove at a predetermined displacement corresponding to a predetermined biasing force, whereafter the rate of increase of biasing force with full displacement is significantly greater than the rate of increase of biasing force with displacement at displacements below the predetermined displacement. When the O-ring is in its normal position corresponding to zero torque on the gear wheel, the O-ring may therefore only partially fill the groove at this point.
According to a second aspect, the invention provides an electric power assisted steering system comprising a housing, an electric motor fixed relative to the housing having a stator and a rotor, and input shaft operatively connected to the rotor, an output shaft operatively connected to a steering column, and a torque sensor adapted to produce an output signal indicative of the torque in the output shaft, the motor being adapted to apply a torque to the output shaft dependent upon the output signal from the torque sensor through a worm gear provided on the input shaft which is adapted to mesh with a wheel gear on the output shaft, the steering system being characterised in that the input shaft is operatively connected to the motor rotor by a flexible coupling, and in which the flexible coupling comprises a resilient spider having a plurality of radially extending arms defining a plurality of substantially radially extending drive faces, one or more of the drive faces co-operating with one or more radial drive faces defined on the rotor and one or more of the drive faces co-operating with drive faces defined on the input shaft.
The input shaft may have a cup formed on its end adjacent the motor rotor. A pin located along the axis of rotation of the rotor may be adapted to be received within the cup. The flexible coupling may be provided between an end face of the cup and the rotor, perhaps around the pin.
The cup may be adapted to receive a first resilient biasing element such as a spring which acts between the end of the pin and the base of the cup to bias the rotor away from the input shaft.
A second compression means may be provided which is adapted to bias the input shaft towards the rotor. This may comprise a spring located between the housing and a bearing means which supports the input shaft.